Heretofore, for example, separators for lithium ion batteries have been produced according to two processes, as broadly divided, of a wet process and a dry process. Of those, the present invention is classified into a wet process. One ordinary production process according to the wet process includes first mixing from about 60 to 80 parts by weight of paraffin which is a plasticizer in a high-molecular polyethylene, and then heating the mixture in a twin-screw extruder at a temperature not lower than the solubilization temperature thereof, followed by cooling on a sheet-forming casting roll to produce a sheet having a phase-separated structure. Next, the sheet is, while heated at a temperature not higher than the melting point thereof, stretched to thereby make the sheet secure air permeability and sheet strength, and thereafter the paraffin is removed using an organic solvent and the sheet is then dried, and finally the sheet is annealed at a temperature slightly higher than the stretching temperature thereof, thereby removing the sheet residual stress and expressing the necessary separator characteristics.
Regarding those proposed for improving the strength and the thermal characteristics of the basic separator characteristics of the separator mentioned above, Patent Document 1 exemplifies a nonaqueous electrolyte battery as well as a separator for nonaqueous electrolyte batteries and a production method thereof. The inorganic powder exemplified in the patent document includes titanium oxide, aluminium oxide, potassium titanate, etc.; and the inorganic fibers mentioned therein are those having an average fiber diameter of from 0.1 to 20 μm, and an average fiber length of from 0.1 to tens mm. With those, the patent document describes the effect of improving the separator characteristics. Similarly, Patent Document 2 shows an example of a glass fiber fabric-reinforced microporous polyolefin film with glass fibers compounded therein; Patent Document 3 shows a separator for lithium ion secondary batteries and a battery using the separator, in which an inorganic filler is applied to a nonwoven fabric; and Patent Document 4 illustrates a separator for batteries, a production method thereof and a battery, and shows a compound case with polypropylene therein. All of them are cases of improving the mechanical characteristics and thermal characteristics. Patent Document 5 provides a nanofiber production method, nanofibers, mixed nanofibers, a compounding method, a compound material and a molded article, relating to a process and an apparatus for cellulose production applicable to the present patent application.
Here, the basic functions of conventional separators are described. In a lithium ion battery, the separator is positioned between a positive electrode and a negative electrode, and exists in a state of holding an electrolyte in the open micropores therein. When given a load, the lithium ions in the positive electrode are deionized into the electrolyte while the electrons are left, then reach the negative electrode after having passed through the micropores of the separator, and are thus stored between the carbon lattices. At this time, the electrons are transferred to the negative electrode through the circuit, but the separator must be an insulator so as to prevent short-circuiting between the positive and negative electrodes. In addition, the separator for use in the lithium ion battery is required to have the ability not to prevent the ion conduction between the both electrodes, to hold an electrolyte therein, and to be resistant to the electrolyte. For preventing the separator from being broken owing to the pressure given thereto in electrode winding, or owing to the pressure also given thereto through expansion and contraction of electrodes in charging/discharging, or owing to the impact given thereto in falling of batteries, the separator is further required to have a high puncture strength. The high puncture strength is important for the reason that, when a lithium ion battery is degraded with time, lithium precipitates on the carbon negative electrode and crystallizes like needles thereon, thereby puncturing the separator to be in contact with the positive electrode to cause short-circuiting, and further causes a runaway risk owing to abnormal heat generation.